The present invention relates to a novel oxide superconductor. In particular, the present invention relates to an oxide superconductor consisting of an oxide which comprises, as constituent metal elements, bismuth, lead, strontium, calcium and copper, and has excellent critical current characteristics in a magnetic field.
The novel oxide superconductor of the present invention finds a wide variety of applications such as magnets, cables, various devices, etc.
Hitherto, a number of oxide-base high temperature superconductors have been found. However, all such superconductors have a structure in which superconductive layers consisting of copper-oxygen planes, and non-superconductive layers are alternately laminated, and their electrical magnetic properties have a large anisotropy reflecting such a structure. The crystal systems of such oxide superconductors are tetragonal or orthorhombic, and the copper-oxygen planes form ab planes. A large superconductive current can flow in the direction of such planes, while little current flows in the direction of the c-axis.
Large-sized oxide-base high temperature superconductive materials consist of polycrystals. In such a case, to flow a large superconductive current, the ab planes of the crystals should be aligned, that is, the crystals are oriented, and the current is allowed to flow in parallel with the ab planes.
Bismuth-base superconductors have the very large anisotropy of crystal growth rates, and thus the ab planes of the crystals, in which a superconductive current easily flows, grow in the form of a thin plate. Therefore, it is easy to mechanically orient crystals, or to orient crystals during the growth of crystals from a melt by making use of the anisotropy of the surface or interface energy.
Among bismuth-base superconductors, the Bi2212 and Bi2223 phases, which have high critical temperatures (Tc), have been developed as industrial materials. The ratio of bismuth:strontium:calcium:copper in the latter phase is about 2:2:1:2, while such a ratio in the latter phase is about 2:2:2:3. In the latter phase, 10 to 20 atomic % of lead is replaced with bismuth for the acceleration of the crystal growth and the stabilization of the structure.
The development of wires using such bismuth-base superconductive materials is one of the most advanced areas among the oxide high temperature superconductive materials, because of the easy orientation of crystals. In the case of the Bi2212 phase, a wire, which has a thickness of several ten to several hundred micrometers and consists of plate-form crystals having dense and oriented structures, has been developed by a melting-solidification process on a tape-formmetal substrate or in a metal sheath. In the case of the Bi2223 phase, a tape-form metal sheathed wire has been developed, in which the crystals are mechanically oriented in the course of a cold work.
The critical current density (Jc) of such a wire in the absence of a magnetic field is several ten thousand A/cm2 at the liquid nitrogen temperature (77K), or several hundred thousand A/cm2 at a low temperature of 30K or less. These values are on the highest level among the oxide high temperature superconductors having a length of several meters or longer, and come up to a practical level. At present, current leads, superconductive cables, superconductive magnet devices which can operate at 20K or less, and the like are developed using such wires, and come into practical use.
The characteristics of superconductive materials from the viewpoint of applications in the heavy electric industry are that a large amount of a current is allowed to flow through the materials at a high current density without loss and, in this connection, they can generate a strong magnetic field of several teslas or larger. Conventional (classical) metal superconductors inevitably require cooling with liquid helium which is expensive and a valuable resource, since they have a low Tc. However, oxide high temperature superconductors can find applications which require economical and easy cooling with liquid nitrogen or by refrigerators. Therefore, it is expected that superconductive equipment will widely spread. Furthermore, oxide high temperature superconductors are expected to find many applications in the technical fields, which are in a magnetic field or which generate a magnetic field, rather than applications such as cables, etc. in the absence of a magnetic field.
As stated above, bismuth-based superconductors have a large anisotropy of crystal growth rates and thus their oriented structures can be easily attained. Thus, their application as materials are most advanced, and it may be possible to greatly expand the market of superconductors, if the applications at high temperature in a magnetic field are developed using bismuth-based superconductors.
However, among oxide superconductors, bismuth-based superconductors are typical materials having a very large electrical magnetic anisotropy, that is, a very weak bond between superconductive layers. Thus, a magnetic flux is hardly pinned in a superconductor in a magnetic field in parallel with the c-axis of a crystal, and the magnetic flux is shifted even by a little current, so that a finite resistance is generated in the superconductor. The influence of thermal fluctuation is small, and pinning effectively functions at a low temperature, and superconductors have a sufficiently high Jc (critical current density) in a high magnetic field. However, at a temperature of 30K or higher, a magnetic flux very easily moves, and thus Jc sharply drops in a magnetic field. Accordingly, bismuth-based superconductors exhibit practically sufficient Jc only in a very small magnetic field at a high temperature such as the liquid nitrogen temperature. For example, in the case of a Bi2212 phase wire, the intensity of a magnetic field at which Jc becomes 0 (zero) (irreversible magnetic field) is only 0.02 tesla in a magnetic field in parallel with the c-axis at the liquid nitrogen temperature. In addition, in the case of other oxide superconductors having a stronger pinning function, a material having a long length and high Jc has not been developed, since it is difficult to control the orientation of crystals.
Many attempts have been made to improve the pinning property of bismuth-based superconductors. The introduction of artificial defects by the irradiation with heavy ions or neutrons, which is most effective, is not an industrially applicable method, while the inclusion of Ti, Zr, Hf, etc. in crystals cannot improve the characteristics of superconductors to practical levels.
It is necessary to introduce effective pinning centers to improve the Jc characteristics of bismuth-based superconductors in a magnetic field. From the practical viewpoint, chemical or mechanical methods are desirable to introduce pinning centers. Examples of pinning centers include non-superconductive deposits, structural defects, grain boundaries of crystals, etc. Since the crystals of bismuth-based superconductors are in the form of thin plates, it is difficult to intentionally add impurities to the superconductors and introduce conductive deposits in the inside of crystals, while the grain boundaries function as weak pinning centers.
Yttrium- or lanthanum-based superconductors have weak superconductivity around oxygen defects, and the superconductivity is broken near the oxygen defects in a magnetic field having a certain intensity. Such a phenomenon is used to improve a pinning power. However, in the case of bismuth-based superconductors, an essential pinning function is not improved, although anisotropic electrical magnetic characteristics are more or less improved by controlling the amount of oxygen.
The present inventors have found that, when a part of bismuth atoms are replaced by lead atoms in the Bi2212 phase of a bismuth-based superconductor, domains having a high lead concentration and domains having a low lead concentration form throughout crystals so that plate-form or columnar internal structures form at a lead-rich composition in which a molar ratio of lead to bismuth is 0.2:1 or larger.
According to the first aspect of the invention, there is provided an oxide superconductor consisting of an oxide which comprises, as constituent metal elements, bismuth, lead, strontium, calcium and copper having a molar ratio of lead to bismuth of at least 0.2:1, preferably from 0.2:1 to 0.7:1, and has a high irreversible magnetic field and a high critical current density.
According to the second aspect of the present invention, there is provided an oxide superconductor consisting of an oxide which comprises, as constituent metal elements, bismuth, lead, strontium, calcium and copper, and has a domain structure comprising domains M having a long-period structure with a period of at least 30 xc3x85, preferably from 30 to 80 xc3x85, in the direction of the b-axis, and domains N having no long-period structure.
Preferably, the oxide superconductors according to the second aspect of the invention has
(A) a layered internal structure in which plate-form crystalline domains N having a thickness of about 50 to 1,000 xc3x85 and a relatively high lead concentration, and plate-form crystalline domains M having a thickness of about 100 to 2,000 xc3x85 and a relatively low lead concentration are alternately laminated in the direction of the b-axis of the crystal, or in a direction which slants from the b-axis towards the a-axis (for example, a direction which slants by 0 to 60 degrees from the b-axis),
(B) a columnar internal structure in which columnar or cylindroidal crystalline domains having a diameter of about 100 to 2,000 xc3x85 and a relatively high lead concentration are present at an interval of about 100 to 2,000 xc3x85 in a matrix crystalline domain M having a relatively low lead concentration, or
(C) both the layered internal structure (A) and the columnar internal structure (B).
In each case, the lead concentration in the domains N is higher than that in the domain or domains M.